So first of all... I hope everyone is having a wonderful Thanksgiving!!!
As far as the Miata suspension goes, the short version of the advice is dependent on what you're going for. If you just want the car to be lower, then springs will absolutely do the trick and you need not read further. Otherwise, I'll save you some time searching on Miata.net and give you what I think are the best options.
I have several problems with the H&R race springs. First, they claim to lower the car 1.5", which is significant on the Miata. To lower the car that much you should be at least doubling the spring rate to prevent bottoming. The H&Rs are only 30-40% stiffer than the OE MSM springs. So, given that you'll be riding on the bumpstops a majority of the time in which case handling and ride quality will both suffer. At the end of the day it is all about travel, and with just H&R springs you lose travel and that is not ideal on a Miata. The other big problem I have with the H&Rs is the spring rates are too close together at 320/275. Miatas like 1.5-2x more spring rate in the front of the car. Just putting H&Rs on a MSM that is otherwise stock would make it very loose. You'd either have to get a bigger front bar, or unhook the rear bar to bring it back to a manageable amount of oversteer.
I wouldn't waste my money on H&R springs, even if it was a temporary solution. The OE suspension is just fine on the MSM and keeping it stock doesn't mess with the geometry or balance at all. Which for a new to you car, is important.
So... Good options time!
The Bilsteins on the MSM are nothing to scoff at. They could easily be the base of a really good suspension setup, but they would have to be revalved to handle higher spring rates. Coilover sleeves can be purchased for the Bilsteins as well:
http://fatcatmotorsports.com/FCM_Bilstein_mounts_NA.htm (they can be had for cheaper I'm sure, but I'm not sure which of these would be needed
http://www.race-mart.com/Coil-Over-Kits_c_5888.html).
Bilstein revalve ($300, which is a guess) + sleeves ($250) + springs ($300) = roughly $850. In my opinion this is the cheapest option to a really great dual purpose setup. You can pick any spring rate you want (550-350 would be awesome), and you would then have shocks valved specifically to match that rate. But you don't even need to take my word for it... Call Shaikh at FCM and get his advice. He knows a great deal about Miata suspensions and is the man when it comes to revalving Bilsteins.
Otherwise... Flyin' Miata V-Maxx is the only entry level setup worth considering. I think they're in the $600-700 range depending on how they're configured. I've honestly never met anyone who has used these. But for a budget setup they are widely considered to be the best.
Koni Sport (Yellows) and/or Race shocks with Ground Control sleeves and springs is an incredibly popular option. Mostly because it is a budget autocross/track setup which lots of adjustability. You get to pick the spring rate you go with. But this setup is probably $800-1000. Once it was all said and done I had about $1200 invested in my Koni/GC setup between the shocks, springs, bumpstops, and top hats.
Based on what Jess posted, it looks like Emilio will have another good entry level option.
